RU2473926C1 - APPARATUS FOR DETERMINING ABSORBED DOSE OF β-RADIATION IN SOLID-STATE THERMOLUMINESCENT DETECTOR - Google Patents

Abstract

FIELD: physics.

SUBSTANCE: apparatus for determining absorbed dose of ionising β-radiation in a thermoluminescent detector has a unit for heating said detector and a unit for detecting thermoluminescence of said detector; the output of the unit for detecting thermoluminescence is connected to the input of a unit for estimating absorbed dose from parameters of the obtained thermoluminescence curve, wherein between the unit for detecting thermoluminescence and the thermoluminescent detector on the propagation path of light from said detector there is a unit for selecting wavelengths of the detected thermoluminescence, wherein the solid-state thermoluminescent detector used is monocrystalline aluminium nitride AlN, the unit for selecting wavelengths of the detected thermoluminescence has characteristics which provide a wavelength selection function only within the range from 340 to 380 nm.

EFFECT: wider range of linearity of the dose curve, high accuracy of estimating absorbed dose of β-radiation, wider field of use of the apparatus, wider range of apparatus of determining absorbed dose of ionising β-radiation in a solid-state thermoluminescent detector.

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Description

The invention relates to radiation physics, and in particular to devices for estimating the accumulated dose of ionizing β-radiation using solid-state thermoluminescent detectors, and can be used in personal and clinical dosimetry, when monitoring the radiation situation in various conditions.

The registration and measurement of the absorbed dose of radiation of various types is carried out using solid-state thermoluminescent detectors made of various materials, such as lithium fluoride LiF, calcium fluoride CaF ₂ , aluminum oxide Al ₂ O ₃ , calcium sulfate CaSO ₄ : Dy; Studies are underway to create radiation-sensitive media based on substances of different classes, in particular wide-gap oxide materials (BeO, MgO, SiO ₂ ) [V.S. Kortov, I.I. Milman, S.V. Nikiforov, Solid State Dosimetry. Proceedings of TPU, 2000, T.303, issue 2, p. 35-45].

However, there is a need for the use of solid-state detectors with enhanced tissue equivalence and better suitability for use in personal dosimetry, including the evaluation of the absorbed dose of β radiation when working with radioactive materials and in space conditions.

Known material such as aluminum nitride AlN, direct gap material with a large band gap [L.I. Berger, Semiconductor materials, CRC Press, 1997, pp.123-124]. It is a heat-resistant, acid-resistant material suitable for use in high-temperature semiconductor devices in polycrystalline form. Due to the unique combination of physical and electrical characteristics: high thermal conductivity, good electrical insulation properties, moderate coefficient of thermal expansion at a relatively low cost, aluminum nitride is used as a structural ceramic material in the manufacture of cases and substrates of integrated circuits, powerful transistors, absorbers, and end loads, including in space technology [V.I. Kostenko, V.S. Seregin, L.A. Groshkova, A.I. Vasilevich, Modern Information and Design Office Technologies, 2003, http://www.iki.rssi.ni/seminar/tarusa200406/3-19.pdf].

As shown by studying the reflection and excitation spectra of blue luminescence of AlN crystals in the energy range 3–40 eV, it is possible to use crystalline aluminum nitride in optoelectronics as photodiodes in the ultraviolet region of the spectrum [Michailin VV, Oranovskii VE, Pacesova S., Pastrnak J., Salamatov AS, Physica Status Solidi (b) 58 (1973) K51].

It is known [Radiation Measurements, Volume 33, Issue 5, October 2001, Pages 731-735] that ceramic material in the form of aluminum nitride doped with yttrium oxide (AlN-Y ₂ O ₃ ), when irradiated with ultraviolet light and determine the absorbed dose by a thermoluminescent method, has linear dose dependence. The possibility of using ceramic AlN-Y ₂ O ₃ for dosimetry of ultraviolet radiation was discussed. However, no systematic studies of the possibility of measuring the absorbed dose of beta radiation were performed. Aluminum nitride with yttrium has a reduced tissue equivalence to radiation. The above results associated with the irradiation of doped yttrium oxide ceramic aluminum nitride with ultraviolet light cannot be applied to pure aluminum nitride and to materials irradiated with particle radiation, in particular β radiation.

A device for determining the absorbed dose of ionizing radiation [V. Shtolts, R. Bernhard, Dosimetry of ionizing radiation, per. from English, Riga, Zinatne, 1982, pp. 97-98, Fig. 22], including a dosimetric sensor (thermoluminescent crystallophosphorus), a heating unit for the indicated dosimetric sensor, and a recording unit (measuring intensity) of a thermoluminescent glow based on a photo-electron multiplier with an amplifier signal converter, thermoluminescence curve recorder and counting-printing device. The device provides a measurement of the intensity of thermoluminescent radiation (emission) at any time in the entire range of spectral sensitivity of the photoelectronic multiplier (different types of multipliers), that is, in a wide frequency range of thermoluminescent radiation, including, in particular, the entire visible radiation spectrum, from 160 to 1200 nm (with a width of at least 85 nm) [Aksenenko MD, Baranochnikov ML, Optical radiation receivers. Handbook, M.: Radio and Communications, 1987, p. 17, Table 2.1]. That is, in the known device for measuring the intensity of the thermoluminescent glow and estimating the absorbed dose, wavelength separation blocks of the recorded thermoluminescent glow are used, operating in a wide region of the visible spectrum, providing measurements in a wide, non-specific predetermined region of the spectrum.

The disadvantage of the analogue under consideration [V. Shtolts, R. Bernhard, Dosimetry ... 1982, pp. 97-98, Fig. 22] is the presence of nonlinearity (superlinearity) of the dose dependence for absorbed doses of more than 0.192 ÷ 0.32 Gy, as evidenced by the results studies [BCKortov et al. Solid State Physics, 2006, Volume 48, Issue 3, pp. 421-426, Fig. 1]. The influence of the superlinearity of the dose dependence leads to a distortion of the result of the assessment of the absorbed dose and is the reason for the decrease in the accuracy of determination (assessment) of the absorbed dose at absorbed doses in excess of 0.192 ÷ 0.32 Gy. The range of doses recorded is limited.

A device is also known for determining the absorbed dose of ionizing β-radiation [I. I. Milman, S. V. Nikiforov, B. C. Kortov, A. K. Kilmetov, Quality control of radiation detectors for radiation defectoscopy. Defectoscopy, 1996, No. 112, pp. 64-70, Fig. 1], including a thermoluminescent detector based on an anion-defective aluminum oxide single crystal (TLD-500K), a heating unit for the indicated thermoluminescent detector, a unit for registering the glow of a thermoluminescent detector in the form of a photoelectron multiplier with an analog-to-digital converter and a pulse counter, as well as an absorbed dose estimation unit based on IBM PC computers. Between the mentioned recording unit and the thermoluminescent detector, on the propagation path of the glow of this detector, there is a wavelength separation unit of the recorded thermoluminescent glow, made in the form of a replaceable filter or a monochromator. The replaceable filter is designed to highlight a certain range of wavelengths that are not specified in the prototype device, which are displayed by a thermoluminescent detector. The monochromator also ensures the isolation of wavelengths not specifically specified in the device in question with a spectrum width of ± 1 ÷ 3 nm. If the characteristics of the replaceable filter and monochromator are randomly selected with respect to the range (wavelength) of the recorded thermoluminescent glow, there will be a random, undefined error in the estimate of the absorbed dose, which will lead to unpredictable errors, which is unacceptable from the point of view of measurement theory and is not used. When conducting measurements in a wide range of the visible spectrum, the disadvantage of the prototype device under consideration is, as for the previous analogue, a decrease in the accuracy of determination (estimation) of the absorbed dose at higher values of the absorbed dose (more than 0.192 ÷ 0.32 Gy) due to the influence of nonlinearity of the dose dependence, and also limiting the area of use of the device at higher absorbed doses.

The prototype of the proposed invention is a device for determining the absorbed dose of β-radiation in a thermoluminescent detector based on an anion-defective single crystal of aluminum oxide [RF patent 2378666], including a heating unit for the specified detector, as well as a unit for detecting the thermoluminescent glow of this detector, the output of the thermoluminescent glow recording unit is connected with the input of the absorbed dose estimation unit according to the parameters of the obtained thermoluminescence curve, and between the registration unit is thermoluminescent and luminescence of the thermoluminescent detector in the path of the luminescence of said detector located block allocation of wavelengths detected to thermoluminescent glow characteristics provide isolation function only wavelengths within the range of 500 to 570 nm.

The prototype device provides a linear dose dependence with absorbed doses up to 1 Gy. The disadvantages of the device are the limited linearity range of the dose dependence, the decrease in the accuracy of determination (estimation) of the absorbed dose at values exceeding 1 Gy, and the dependence of the parameters of the thermoluminescent peaks on the dose and the conditions for preliminary photothermal treatment of the material [IIMilman, VSKortov, SVNikiforov, Radiation Measurements , 1998, Vol.29, No 3-4, pp.401-410]. The scope of the method is limited to higher absorbed doses.

The objective of the invention is to expand the range of linearity of the dose dependence and the corresponding increase in the accuracy of estimating the absorbed dose of β-radiation, reducing the dependence of the parameters of thermoluminescent peaks on the dose and conditions of the pre-photothermal treatment of the material, expanding the scope of the method, expanding the arsenal of methods for determining the absorbed dose of ionizing β-radiation in solid state thermoluminescent detector.

To solve the problem, a device for determining the absorbed dose of ionizing β-radiation in a thermoluminescent detector, including a heating unit for the specified detector, as well as a unit for detecting the thermoluminescent glow of this detector, the output of the thermoluminescent glow recording unit is connected to the input of the absorbed dose estimating unit according to the parameters of the obtained thermal emission curve, moreover, between the registration unit of the thermoluminescent glow and the thermoluminescent detector on the path of the distribution of candles For this detector, there is a block for extracting the wavelengths of the detected thermoluminescent glow, characterized in that monolithic aluminum nitride AlN is used as a solid-state thermoluminescent detector, and a block for isolating the wavelengths of the recorded thermoluminescent glow is made with characteristics providing the function of extracting wavelengths only within the range from 340 to 380 nm.

The technical result of the invention is an increase in the upper value of the linear range of the dose dependence up to 5 Gy (Fig. 1) when measuring the absorbed dose of ionizing β-radiation. This is ensured by the implementation of a set of distinctive and restrictive features of the device, in particular, the use of aluminum nitride single crystal as a solid-state detector and the implementation of a wavelength separation unit of a detected thermoluminescent glow with characteristics providing the function of extracting wavelengths only within the range from 340 to 380 nm. EFFECT: increase of the upper boundary of the linear range of the dose dependence from 1 to 5 Gy, by 5 times in comparison with the prototype. The dependence of the position of the maxima of thermoluminescent peaks on the dose is reduced. The conditions for preliminary photothermal processing of the detector material have a weak effect on its sensitivity.

The proposed invention expands the arsenal of previously known devices for measuring the absorbed dose of ionizing β-radiation, improves the accuracy of estimating the absorbed dose in the dose range from 1 to 5 Gy and expands the scope of the device in the direction of measuring increased values of the absorbed dose.

When using the wavelength separation unit of the detected thermoluminescent glow with wavelengths less than 340 nm, the upper value of the linear range of the dose dependence is significantly reduced. With an increase in the wavelengths transmitted by the indicated unit over 380 nm, the components of the thermoluminescent glow of the detector are recorded, introducing errors in the estimate of the absorbed dose and significantly reducing the accuracy of such an estimate. Such components are caused by uncontrolled impurities of the material of the thermoluminescent detector, the thermal background, and the influence of deeply located traps of a single crystal of aluminum nitride.

The described relationship between the distinguishing features of the proposed invention and the new technical result is experimentally identified by the inventors.

The invention is illustrated by drawings:

figure 1 - obtained by the authors of the dose dependence with linearity that occurs with absorbed doses up to 5 Gy; this figure shows the dependences of the radiation intensity and light sum on the absorbed dose obtained for single-crystal aluminum nitride by the method of thermal stimulation; the horizontal axis represents the absorbed dose (Gy); values of radiation intensity in relative units (rel. units) are plotted on the left vertical axis, light sums (rel. units) on the right vertical axis;

figure 2 is a block diagram of a device for determining the absorbed dose of β-radiation in a thermoluminescent detector based on a single crystal of aluminum nitride;

figure 3 - obtained by the authors of the dependence of the intensity of thermoluminescence on the heating temperature at two values of the absorbed dose (1.92 and 3.84 Gy); temperature values (° C) are plotted along the horizontal axis, and thermoluminescence intensities in relative units (rel. units) are plotted along the vertical axis.

A device for determining the absorbed dose of β-radiation (figure 2) includes a solid-state thermoluminescent detector 1 based on a single crystal of aluminum nitride AlN, a heating unit 2 of the specified detector 1 and a unit 3 for recording the thermoluminescent glow of the same detector 1. The output 4 of the block 3 for recording the thermoluminescent glow is connected with an input of 5 block 6 absorbed dose assessment. Between the mentioned recording unit 3 and the thermoluminescent detector 1, on the propagation path of the glow 7 of this detector, there is a wavelength separation unit 8 of the detected thermoluminescent glow, indicated in FIG. 3 as a filter. Block 8 of the selection of wavelengths of the recorded thermoluminescent glow is made with characteristics that provide the function of selecting wavelengths only within the range from 340 to 380 nm (glow 9).

The heating unit 2 includes a heating table on which the detector 1 is placed, and a heating power adjustment device (not shown).

Detector 1 is a sample of single-crystal aluminum nitride having a wurtzite type of lattice, specific resistance 10 ¹¹ ÷ 10 ¹³ Ohm · cm, thermal conductivity 3.2 W / (cm · K), crystal cell parameters and thermal expansion coefficient close to gallium nitride, as well as a dislocation density of less than 10 ³ cm ^-2 .

The wavelength separation unit 8 of the recorded thermoluminescent glow 7 is an optical glass filter (for example, UFS-8 type) that performs the function of isolating (passing through itself) the wavelengths of the thermoluminescent glow in the range 340–380 nm (glow 9). As block 8, an appropriate interference filter can be used.

Block 3 registration thermoluminescent glow 9 is a photomultiplier tube, for example, type FEU-39A with an amplifier and signal converter (not shown).

Unit 6 for estimating the absorbed dose (not shown) is a microprocessor or personal computer (computer) with an interface for receiving a signal from block 3 for registering the thermoluminescent glow of detector 1. Block 6 performs the functions of setting the temperature values of detector 1, determining the values of the intensity of thermoluminescent glow 9 at specified values temperature, constructing a thermoluminescence curve (dependence of the intensity of the thermoluminescent glow 9 on the heating temperature of the detector 1), determining the value of light osumma of the specified curve and estimates of the absorbed dose by the obtained value of the light sum. The absorbed dose can also be estimated by the peak intensity of the thermal emission curve.

To control the heating of the detector 1, a control unit (not shown) is used, the inputs and outputs of which are connected to the power control device of the heating unit 2 and through the appropriate interface to a microprocessor or personal computer of the absorbed dose estimation unit 6. The function of said control unit may be performed by said microprocessor itself (personal computer).

In the computer of block 6, for measuring the absorbed dose, control programs for the measuring system, registration of thermal emission curves, and mathematical packages, in particular Excel or Origin, are used.

A device for determining the absorbed dose of β-radiation in a thermoluminescent detector based on single-crystal aluminum nitride works as follows.

The measured sample 1 of aluminum nitride (detector 1, figure 2) before starting the measurement has room temperature. The general technical concept of room temperature includes a temperature range of 20 ÷ 25 ° C, but a range of 17 to 30 ° C can also be used. To determine the desired value of the absorbed dose of β-radiation, sample 1, if necessary, is heated to the first selected temperature value, for example 25 ° C. The first temperature selected can be the room temperature that is valid in the room. Using a filter 8, a glow 9 in the range 340–380 nm is isolated from the thermoluminescent glow 7 of this sample 1. Using blocks 3 and 6, determine the intensity of the thermoluminescent glow at a set temperature and build the first point of the desired curve of thermal emission. Next, the detector 1 is linearly heated to the following temperature values and, after set time periods (e.g., 1 s), the subsequent points of the desired thermal emission curve are similarly constructed until the temperature limit of sample 1, equal to, for example, 400 ° С or more, is reached. Heating is carried out at a speed selected in the range from 0.2 to 10 ° C / s, for example 2 ° C / s. When the values of the final heating temperature are less than 400 ° C, the accuracy of the estimate of the absorbed dose decreases. For example, at a final temperature value of 380 ° C, the corresponding part of the area under the curve of the dependence of the intensity of thermoluminescence on temperature, which is to the right of the temperature value of 380 ° C (figure 3), is not taken into account.

Data on the time elapsed since the beginning of the measurements, the temperature of the sample 1 and the intensity of its thermoluminescent glow 9, obtained using the described device, are recorded in the data file. The data file is processed in a mathematical package, for example, such as Excel or Origin. From the desired heat-emission curve obtained, the desired light sum value or the desired peak intensity value of the specified curve is determined, from which the value of the desired dose of β radiation absorbed by sample 1 is estimated. For this, the measured sample 1, prepared for subsequent use (freed from the previously received absorbed dose of β-radiation), is exposed to a known reference value of the dose of β-radiation (of the order of 0.01 ÷ 0.05 Gy). Then, in the manner described above, the value of the reference light sum or the reference intensity of the peak of the thermal emission curve is determined. The desired value of the absorbed dose of β-radiation of sample 1 is calculated using block 6 estimates of the absorbed dose according to the following formulas:

or

,

,

D _claim wherein - the desired value of the absorbed doses of β-radiation, Gy;

D _reference - the reference value of the absorbed dose of β-radiation, set within 0.01 ÷ 0.05 Gy;

S _claim - the desired value of the desired curve lightsum thermoluminescence, rel. units;

S _reference - the reference value of the light sum of the reference curve of thermal emission, rel. units;

I _claim - the desired value of the peak intensity of the thermoluminescence of the desired curve, rel. units;

I _reference - the reference value of the intensity of the peak of the reference curve of thermal emission, rel. units

The table shows the results of measurements and estimates of the absorbed dose of β-radiation using aluminum nitride single crystal AlN as detector 1 at three values of the test absorbed dose (0.0016 Gy, 0.48 Gy, and 3.84 Gy). The reference value of the absorbed dose of β-radiation was taken equal to 0.03 Gy. The test and reference values of the absorbed dose in these samples were established by irradiating these samples at room temperature with β radiation of a ⁹⁰ Sr / ⁹⁰ Y source with a dose rate of 0.032 Gy / min at the sample location. The heating rate of sample 1 was 2 ° C / s. The values of the errors in estimating the required absorbed dose in percent relative to the standard absorbed dose are given. The permissible relative error in estimating the required absorbed dose is 15% (GOST 8.035-82 “GSI. State primary standard and state calibration scheme for measuring absorbed dose and absorbed beta dose rate”).

Examples 1, 2, 3 of determining the absorbed dose of ionizing β-radiation in a solid-state thermoluminescent detector based on aluminum nitride AlN are described below, numbered according to the rows of the table (from top to bottom). The total estimation error in the table is defined as the root of the sum of the squares of the errors in light sum and intensity, also presented in the table.

Example 1

The absorbed dose of ionizing β-radiation in a solid-state thermoluminescent detector 1 based on monocrystalline aluminum nitride AlN is determined by heating the irradiated detector 1 at a rate of 2 ° C / s, starting from room temperature (25 ° C) and ending with a temperature of 420 ° C. The intensity of the thermoluminescent glow is measured only within the wavelength range from 340 to 380 nm, with the desired absorbed dose of the irradiated detector 1 equal to 0.0016 Gy. As a result, the total error in the estimate of the desired absorbed dose is 2.5%, that is, it is permissible (less than 15%).

Example 2

The absorbed dose of ionizing β-radiation in a solid-state thermoluminescent detector 1 based on monocrystalline aluminum nitride AlN is determined in the same way as in example 1, with the exception of heating the detector 1 to a temperature of 400 ° C, with the desired absorbed dose of the irradiated detector 1 equal to 0.48 Gr. The total error in estimating the absorbed dose is 10.4%, which is acceptable.

Example 3

The absorbed dose of ionizing β-radiation in a solid-state thermoluminescent detector 1 based on monocrystalline aluminum nitride AlN is determined in the same way as in example 1, with the desired absorbed dose of irradiated detector 1 equal to 3.84 Gy. The total error in the estimate of the required absorbed dose is 9.2% and is acceptable.

Claims (1)

A device for determining the absorbed dose of ionizing β-radiation in a thermoluminescent detector includes a heating unit for the specified detector, as well as a unit for recording the thermoluminescent glow of this detector, the output of the registration unit for the thermoluminescent glow is connected to the input of the unit for estimating the absorbed dose by the parameters of the obtained thermal emission curve, and between the registration unit for thermoluminescent luminescence and thermoluminescent detector on the propagation path of the luminescence of said detector is located en block allocation wavelengths detected thermoluminescent glow, characterized in that as a solid-state thermoluminescent detector used monocrystalline aluminum nitride AlN, block allocation wavelengths detected thermoluminescent glow configured with characteristics that provide the function of allocating wavelengths only within a range of 340 to 380 nm.

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